People with interesting brains:

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Here about the beach I wander’d, nourishing a youth sublime
With the fairy tales of science, and the long result of Time…

—Tennyson

Temporal coincidence plays havoc with our ideas about other civilizations in the cosmos. If we want to detect them, their society must at least have developed to the point that it can manipulate electromagnetic waves. But its technology has to be of sufficient strength to be noticed. The kind of signals people were listening to 100 years ago on crystal sets wouldn’t remotely fit the bill, and neither would our primitive TV signals of the 1950s. So we’re looking for strong signals and cultures older than our own.

Now consider how short a time we’re talking about. We have been using radio for a bit over a century, which is on the order of one part in 100,000,000 of the lifespan of our star. You may recall the work of Brian Lacki, which I wrote about four years ago (see Alpha Centauri and the Search for Technosignatures). Lacki, now at Oxford, points out how unlikely it would be to find any two stars remotely near each other whose civilization ‘window’ corresponded to our own. In other words, even if we last a million years as a technological civilization, we’re just the blink of an eye in cosmic time.

Image: Brian Lacki, whose work for Breakthrough Listen continues to explore both the scientific and philosophical implications of SETI. Credit: University of Oxford.

Adam Frank at the University of Rochester has worked this same landscape. He thinks we might well find ourselves in a galaxy that at one time or another had flourishing civilizations that are long gone. We are separated not only in space but also in time. Maybe there are such things as civilizations that are immortal, but it seems more likely that all cultures eventually end, even if by morphing into some other form.

What would a billion year old civilization look like? Obviously we have no idea, but it’s conceivable that such a culture, surely non-biological and perhaps non-corporeal, would be able to manipulate matter and spacetime in ways that might simply mimic nature itself. Impossible to find that one. A more likely SETI catch would be a civilization that has had space technologies just long enough to have the capability of interstellar flight on a large scale. In a new paper, Lacki looks at what its technosignature might look like. If you’re thinking Dyson spheres or swarms, you’re on the right track, but as it turns out, such energy gathering structures have time problems of their own.

Lacki’s description of a megaswarm surrounding a star:

These swarms, practically by definition, need to have a large number of elements, whether their purpose is communication or exploitation. Moreover, the swarm orbital belts need to have a wide range of inclinations. This ensures that the luminosity is being collected or modulated in all directions. But this in turn implies a wide range of velocities, comparable to the circular orbital velocity. Another problem is that the number of belts that can “fit” into a swarm without crossing is limited.

Image: Artist’s impression of a Dyson swarm. Credit: Archibald Tuttle / Wikimedia Commons. CC BY-SA 4.0.

Shards of Time

The temporal problem persists, for even a million year ‘window’ is a sliver on the cosmic scale. The L factor in the Drake equation is a great unknown, but it is conceivable that the death of a million-year old culture would be survived by its artifacts, acting to give us clues to its past just as fossils tell us about the early stages of life on Earth. So might we hope to find an ancient, abandoned Dyson swarm around a star close enough to observe?

Lacki is interested in failure modes, the problem of things that break down. Helpfully, megastructures are by definition gigantic, and it is not inconceivable that. Dyson structures of one kind or another could register in our astronomical data. As the paper notes, a wide variety covering different fractions of the host star can be imagined. We can scale a Dyson swarm down or up in size, with perhaps the largest ever proposed being from none other than Nikolai Kardashev, who discusses in a 1985 paper a disk parsecs-wide built around a galactic nucleus (!).

I’m talking about Dyson swarms instead of spheres because from what we know of material science, solid structures would suffer from extreme instabilities. But swarms can be actively managed. We have a history of interest in swarms dating back to 1958, when Project Needles at MIT contemplated placing a ring of 480,000,000 copper dipole antennas in orbit to enhance military communications (the idea was also known as Project West Ford). Although two launches were carried out experimentally, the project was eventually shelved because of advances in communications satellites.

So we humans already ponder enclosing the planet in one way or another, and planetary swarms, as Lacki notes, are already with us, considering the constellations of satellites in Earth orbit, the very early stages of a mini Dyson swarm. Just yesterday, the announcement by SpinLaunch that it will launch hundreds of microsatellites into orbit using a centrifugal cannon gave us another instance. Enclosing a star in a gradually thickening swarm seems like one way to harvest energy, but if such structures were built, they would have to be continuously maintained. The civilization behind a Dyson swarm needs to survive if the swarm itself is to remain viable.

For the gist of Lacki’s paper is that on the timescales we’re talking about, an abandoned Dyson swarm would be in trouble within a surprisingly short period of time. Indeed, collisions can begin once the guidance systems in place begin to fail. What Lacki calls the ‘collisional time’ is roughly an orbital period divided by the covering fraction of the swarm. How long it takes to develop into a ‘collisional cascade’ depends upon the configuration of the swarm. Let me quote the paper, which identifies:

…a major threat to megastructure lifespans: if abandoned, the individual elements eventually start crashing into each other at high speeds (as noted in Lacki 2016; Sallmen et al. 2019; Lacki 2020). Not only do the collisions destroy the crashed swarm members, but they spray out many pieces of wreckage. Each of these pieces is itself moving at high speeds, so that even pieces much smaller than the original elements can destroy them. Thus, each collision can produce hundreds of missiles, resulting in a rapid growth of the potentially dangerous population and accelerating the rate of collisions. The result is a collisional cascade, where the swarm elements are smashed into fragments, that are in turn smashed into smaller pieces, and so on, until the entire structure has been reduced to dust. Collisional cascades are thought to have shaped the evolution of minor Solar System body objects like asteroid families and the irregular satellites of the giant planets (Kessler 1981; Nesvorn.

You might think that swarm elements could be organized so that their orbits reduce or eliminate collisions or render them slow enough to be harmless. But gravitational perturbations remain a key problem because the swarm isn’t an isolated system, and in the absence of active maintenance, its degradation is relatively swift.

Image: This is Figure 2 from the paper. Caption: A sketch of a series of coplanar belts heating up with randomized velocities. In panel (a), the belt is a single orbit on which elements are placed in an orderly fashion. Very small random velocities (meters per second or less) cause small deviations in the elements’ orbits, though so small that the belt is still “sharp”, narrower than the elements themselves (b). The random velocities cause the phases to desynchronize, leading to collisions, although they are too slow to damage the elements (cyan bursts). The collision time decreases rapidly in this regime until the belt is as wide as the elements themselves and becomes “fuzzy” (c). The collision time is at its minimum, although impacts are still too small to cause damage. In panel (d), the belts are still not wide enough to overlap, but relative speeds within the belts have become fast enough to catastrophically damage elements (yellow explosions), and are much more frequent than the naive collisional time implies because of the high density within belts. Further heating causes the density to fall and collisions to become rarer until the belts start to overlap (e). Finally, the belts grow so wide that each belt overlaps several others, with collisions occuring between objects in different belts too (f), at which point the swarm is largely randomized. Credit: Brian Lacki.

Keeping the Swarm Alive

Lacki’s mathematical treatment of swarm breakdown is exhaustive and well above my payscale, so I send you to the paper if you want to track the calculations that drive his simulations. But let’s talk about the implications of his work. Far from being static technosignatures, megaswarms surrounding stars are shown to be highly vulnerable. Even the minimal occulter swarm he envisions turns out to have a collision time of less than a million years. A megaswarm needs active maintenance – in our own system, Jupiter’s gravitational effect on a megaswarm would destroy it within several hundred thousand years. These are wafer-thin time windows if scaled against stellar lifetimes.

The solution is to actively maintain the megaswarm and remove perturbing objects by ejecting them from the system. An interesting science fiction scenario indeed, in which extraterrestrials might sacrifice systems planet by planet to maintain a swarm. Lacki works the simulations through gravitational perturbations from passing stars and in-system planets and points to the Lidov-Kozai effect, which turns circular orbits at high inclination into eccentric orbits at low inclination. Also considered is radiation pressure from the host star and radiative forces resulting from the Yarkovsky effect.

How else to keep a swarm going? From the paper:

For all we know, the builders are necessarily long-lived and can maintain an active watch over the elements and actively prevent collisions, or at least counter perturbations. Conceivably, they could also launch tender robots to do the job for them, or the swarm elements have automated guidance. Admittedly, their systems would have to be kept up for millions of years, vastly outlasting anything we have built, but this might be more plausible if we imagine that they are self-replicating. In this view, whenever an element is destroyed, the fragments are consumed and forged into a new element; control systems are constantly regenerated as new generations of tenders are born. Even then, self-replication, repair, and waste collection are probably not perfectly efficient.

The outer reaches of a stellar system would be a better place for a Dyson swarm than the inner system, which would be hostile to small swarm elements, even though the advantage of such a position would be more efficient energy collection. The habitable zone around a star is perhaps the least likely place to look for such a swarm given the perturbing effects of other planets. And if we take the really big picture, we can talk about where in the galaxy swarms might be likely: Low density environments where interactions with other stars are unlikely, as in the outskirts of large galaxies and in their haloes. “This suggests,” Lacki adds, “that megaswarms are more likely to be found in regions that are sometimes considered disfavorable for habitability.”

Ultimately, an abandoned Dyson swarm is ground into microscopie particles via the collision cascades Lacki describes, evolving into nothing more than dispersed ionized gas. If we hope to find an abandoned megastructure like this in our practice of galactic archaeology, what are the odds that we will find it within the window of time within which it can survive without active maintenance? We’d better hope that the swarm creators have extremely long-lived civilizations if we are to exist in the same temporal window as the swarm we want to observe. A dearth of Dyson structures thus far observed may simply be a lack of temporal coincidence, as we search for systems that are inevitably wearing down without the restoring hand of their creators.

The paper is Lacki, “Ground to Dust: Collisional Cascades and the Fate of Kardashev II Megaswarms,” accepted at The Astrophysical Journal (preprint). The Kardashev paper referenced above is “On the Inevitability and the Possible Structure of Super Civilizations,” in The Search for Extraterrestrial Life: Recent Developments, ed. M. D. Papagiannis, Vol. 112, 497–504.

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